As structural steel used as a material for auto parts, industrial machinery parts, construction machinery parts, and other machine structural parts, carbon steel for machine structure use and alloy steel for machine structure use have been employed.
To produce parts from these steel materials, in the past, mainly the “hot forging-cutting” process was employed. In recent years, for the purpose of improving the productivity, a switch to the “cold forging-cutting” process has been underway. By employing the “cold forging-cutting” process in this way, a near net shape is achieved by cold forging and the amount of cutting of the material is slashed, so the productivity is improved.
However, in general, cold forging involves a large degree of working, so the problems arise that the working load is high, the tooling life is short, and parts easily crack. Therefore, improving the cold forgeability of the steel materials used as the starting materials, that is, reducing the load at the time of cold forging and suppressing cracking, has become the most important issue at hand.
On the other hand, auto parts, industrial machinery parts, construction machinery parts, and other machine structural parts are required to have high fatigue strength. To achieve high fatigue strength, it is effective to raise the hardness after cold forging. However, if raising the hardness of the starting material steel to try to raise the hardness after cold forging, the cold forgeability is caused to decrease. That is, in starting material steel, it was difficult to achieve both cold forgeability and fatigue strength.
Therefore, to solve such a problem, to raise the fatigue strength of a cold forged part, the practice has been to heat the part to the Ac3 temperature or more after cold forging to quench and temper it or to heat treat it by induction hardening so as to thereby harden the entire part or its surface.
However, with such a method, the hardness of the part becomes higher after heat treatment, so there were the problems that decrease of the machinability was unavoidable and the merit of improvement of productivity due to the cold forging could not be enjoyed.
Therefore, there are so-called “age-hardening steel materials” which are used for applications for increasing hardness by heat treatment after machining without making the hardness unnecessarily high at the time of machining.
PLT 1 discloses art relating to steel for cold forging and nitridation, steel materials for cold forging and nitridation, and cold forged and nitride parts having as their chemical components, by mass %, C: 0.01 to 0.15%, Si: 0.05% or less, Mn: 0.10 to 0.90%, P: 0.030% or less, S: 0.030% or less, Cr: 0.50 to 2.0%, V: 0.10 to 0.50%, Al: 0.01 to 0.10%, N: 0.00080% or less, and O: 0.0030% or less and having a balance of Fe and impurities, satisfying 399×C+26×Si+123×Mn+30×Cr+32×Mo+19×V≤160 or less, 20≤(669.3×log C−1959.3×log N−6983.3)×(0.067×Mo+0.147×V)≤80, 160≤140×Cr+125×Al+235×V, and 90≤511×C+33×Mn+56×Cu+15×Ni+36×Cr+5×Mo+134×V≤170, having a microstructure of a ferrite-pearlite structure, a ferrite-bainite structure, or a ferrite-pearlite-bainite structure and having an area ratio of ferrite of 70% or more, having a content of V in the precipitates by analysis of extracted residue of 0.10% or less, having a core hardness of a Vickers hardness of 220 or more, and having an effective hardened layer depth of 0.20 mm or more.
PLT 2 discloses art relating to steel for cold heading use having as its chemical components, by mass %, C: 0.06 to 0.50%, Si: 0.05% or less, Mn: 0.5 to 1.0% or less, and V: 0.10 to 0.60%, having a total amount of pro-eutectoid ferrite and pearlite of an area ratio of 90% or more, having the pro-eutectic ferrite of an area % of at least an f-value shown by the formula f=100−125[C]+22.5[V], and having an excellent cold workability and where VC precipitates in the pro-eutectoid ferrite.